This invention relates to automatic gain control (AGC) Loop circuits, usually referred to as AGC loops because automatic gain control necessarily requires a feedback loop of some kind. (There are forward type AGCs that do not require a feedback loop.) AGC loops are used in communication systems to improve receiver operation by varying the gain applied to received signals based upon their detected power. AGC loops are needed in most communication systems because, as a practical matter, received signals are always subject to variations in power. The task of gain control is further complicated by the presence of interfering signals.
Designers of communication receivers face a fundamental choice between first-order AGC loops and second or higher-order AGC loops. The loop “order” is a basic characteristic of feedback control loops in general, not just AGC loops, and refers to the number of mathematical integrators in the loop. First-order loops are less complex and are known for their stability, i.e., their ability to converge on a desired steady-state signal value relatively quickly when there is a change in the received signal strength. For certain received signal conditions, however, first-order loops may not perform as well as desired, and in particular may not react fast enough to large or small received signal changes. First order loops are optimized either for speed or accuracy in performance. In the former case, a large loop bandwidth (equivalent noise bandwidth of the loop) in which the loop can react fast to a change in signal power suffers from poor performance. On the other hand, a first order loop with small loop bandwidth is very slow to react, however, it has a superior performance compared with loops with large bandwidths. For example, in the presence of strong interfering signals, they may tend to clip the interfering signal and thus desensitize the relatively weak desired signal. Second-order and higher-order loops have a theoretically more desirable response characteristic for many applications, but are in general not used because of their potential instability.
Ideally, what is needed is an AGC loop that has the stability of a first-order loop but also has the ability to react rapidly in the presence of large signal changes, interference signals, or in a frequency-hopping communications environment. The present invention is directed to these goals.